1. Technology field
The application generally relates to an electronic apparatus with a hidden antenna, and more particularly, to an electronic apparatus including a metal frame of a part of housing for transmitting/receiving electromagnetic signals.
2. Description of Related Art
Currently, wireless communication has become a more popular choice for human beings to communicate with each other. Correspondingly, there have been developed many kinds of wireless communication apparatuses, such as smart cell phones, multimedia players, personal digital assistants (PDAs), and satellite navigators. Almost all of the electronic apparatus capable of wireless transmittance are developed with a concept toward light weight and slimness, so as to become more welcome to the consumers.
Generally, antennas are critical components for electronic apparatus to receive or transmit signals. Typically, most electronic apparatuses equip with monopole antennas or a planar inverted F antenna to achieve a micro antenna or a hidden antenna. The reason for doing so is that fundamental modes of both of these two kinds of antennas resonate at a ¼ wavelength, so as to be capable of diminishing sizes thereof.
Alternatively, loop antennas are also adopted by some electronic apparatuses. Conventional loop antennas have some certain advantages. For example, a balance-fed type loop antenna can advantageously reduce an excitation current on the metal surface, so that the antenna would be less affected by the environment and the metal surface. Alternatively, the loop antennas may be multiple bent so as to reduce the space occupation thereof for being applied in the small size electronic apparatuses. Principle of application of the loop antenna is to be illustrated below, and whether the mode of the loop antenna could be excited is also discussed below from a point of view whether the energy can be transmitted.
FIG. 1 is a schematic diagram illustrating a current distribution of a loop antenna at a full wavelength mode. Referring to FIG. 1, the loop antenna 110 utilizes a coaxial cable 120 having a resistance of 50Ω as a path for signal transmittance. As shown in FIG. 1, directed by the arrow symbols, along a direction of a current flowing through the loop antenna 110, there exist two current zero points, Z1 and Z2. A current flowing through an internal conductor 121 of the coaxial cable 120 flows out the coaxial cable 120. A current flowing through an external conductor 122 of the coaxial cable 120 flows in the coaxial cable 120. In other words, currents flowing through the internal conductor and the external conductor of the coaxial cable 120 flow along directions opposite one to another. The opposite directions of the currents meet the rule of transmission line for transmitting energy, and therefore the coaxial cable 120 is capable of transmitting energy to the loop antenna 110, and thus exciting a full wavelength resonance mode.
FIG. 2 is a schematic diagram illustrating a current distribution of a loop antenna at a half wavelength mode. Referring to FIG. 2, the loop antenna 210 utilizes a coaxial cable 220 having a resistance of 50Ω as a path for signal transmittance. As shown in FIG. 2, as indicated by the arrows showing a direction of a current flowing through the loop antenna 210, there exists only one current zero point Z21. A current flowing through an external conductor 221 of the coaxial cable 220 flows out of the coaxial cable 220. A current flowing through an external conductor 222 of the coaxial cable 120 also flows out the coaxial cable 120. In other words, currents flowing through the internal conductor 221 and the external conductor 222 of the coaxial cable 120 flow along the same direction. The same current direction violates the rule of transmission line for transmitting energy, and therefore the coaxial cable 220 is incapable of transmitting energy to the loop antenna 210, and thus cannot excite a half wavelength resonance mode.
In summary, a loop antenna is typically operated at a full wavelength resonance mode. In such a way, the loop antenna is likely to achieve an impedance matching of 50Ω and obtain better radiation efficiency. However, because the loop antenna adopts a full wavelength mode for operation, it would occupy a larger space within the electronic apparatus, and thus restricting the miniaturization of the electronic apparatus.